Using a single optical link, rather than separate optical links, to carry upstream optical signals and downstream optical signals may be more cost efficient. This is not only because fewer optical fibres are required in order to transport the optical signals, but also because fewer associated components such as splitters, couplers and splice enclosures may be required. Moreover, upgrading existing or “dark” optical fibre installations for single fibre working (i.e. to carry upstream optical signals and downstream optical signals) may advantageously enable capital and labour costs to be reduced, by making more efficient use of existing fibre installations and thus extending the “time to trench” when additional optical fibres must be laid.
However, being able to “protect” an optical link, whereby the upstream optical signals and downstream optical signals may still be conveyed in the event of an optical fibre cut, or other damage, is very important for a variety of applications.
FIG. 1 illustrates a conventional 1+1 protection apparatus for an optical link 10 which is arranged to carry upstream optical signals only (i.e. optical signals travelling in a single direction). The protection apparatus comprises a first port 16 for coupling to the optical link 10. The protection apparatus further comprises a second port 18 for coupling to a further optical link 12, which may be referred to as a protection optical link. The protection apparatus further comprises a third port 20 for receiving upstream optical signals for transmission over the optical link 10. In addition, the protection apparatus comprises a protection switch 14 operable to selectively couple the third port 20 to the second port 18 instead of to the first port 16; i.e. to cause the upstream optical signals to travel over the further optical link 12, instead of over the optical link 10, for example if an optical fibre cut or other damage is detected.
In order to detect such optical link damage, an optical power detector (not shown) is arranged to monitor the optical power received at the first port 16 from the optical link 10. In the event of an optical fibre cut or other damage a significant portion of the upstream optical signals, transmitted from the first port 16, will be reflected back from the cut interface or otherwise damaged optical link portion, and received at the first port 16. Thus, if the optical power detector detects optical power at a level greater than a threshold (indicating reflection at a power greater than during normal operation), then in response a control signal may be triggered to cause the protection switch 14 to couple the third port 20 to the second port 18, instead of to the first port 16.
Thus, in this way, protection of the optical link 10 may be provided in a simple, cost effective and reliable way. If the protection system were a 1:1 protection system, the remote unit may also detect failure of the optical link, simply by detecting loss of signal of the upstream optical signals, received at the remote unit.